The magnetoelastic effect is a phenomenon whereby the magnetic permeability of a ferromagnetic material is changed when it is subjected to mechanical stresses.
Utilizing the above-mentioned effect for measuring mechanical forces and torques is an idea that arose over fifty years ago. In recent years this idea has attracted an increasingly greater interest.
Above all, the new amorphous materials with their very powerful magnetostriction have contributed to this increase, and also the possibility of using this technique for measuring torques in a contactless and very simple manner.
In addition, transducers based on the magnetoelastic effect are characterized by very high resistance with respect to the external environment and by a high signal power and hence low sensitivity to disturbance.
The design of the above-mentioned transducers varies considerably, but, in principle, the following summary can be made.
A body of a ferromagnetic material is subjected to a periodic magnetization by allowing current to pass through an excitation winding.
The most primitive concept then only measures the inductance in the excitation winding in order thereby to obtain a measure of the magnetic permeability of the material and hence a measure of the mechanical stress.
More sophisticated devices make use of a secondary winding for sensing the time rate of change of the induced magnetic flux.
With the aid of a secondary winding, it is also possible to measure the induced flux in a direction transverse to the magnetization, and in a direction making an angle of 45.degree. with the principal stress direction of the mechanical stresses in the loaded body. This is the case with the known transducers PRESSDUCTOR.RTM. and TORDUCTOR.RTM..
Another frequently used measuring principle comprises measuring the induced magnetic flux in several regions or measuring zones which are subjected to different mechanical stresses, and then forming the difference between these fluxes.
The most common method is to use two measuring zones, one of which is loaded with tensile stress and the other with compressive stress in the direction of the magnetic field. This is done, for example, in the patent specifications of EP 0089916 and U.S. Pat. No. 4,506,554.
Another method is to measure the difference between the fluxes which are induced in a loaded and an unloaded zone.
Irrespective of which method is used in order to measure the permeability change in the magnetoelastic material, the magnetic hysteresis curve, i.e. the B-H curve which each point in the material completes during a period of the magnetization, will be changed when the material is magnetized by a static field. This, of course, influences the measurement of the induced fluxes and leads to changes both of the measuring signal in an unloaded transducer, i.e. the zero signal, and of the sensitivity of the transducer to load.
If the transducer is magnetized by a static field to such an extent that the ferromagnetic material approaches saturation, the differential permeability and also the sensitivity of the transducer will be very low. However, the influence remains in connection with considerably lower fields.
To reduce the influence of this disturbance, attempts have been made to screen off external magnetic fields. However, screening off static or slowly varying, i.e. quasi-static, magnetic fields has proved to be a difficult technical problem. This problem may be particularly difficult when attempts are made to screen off such magnetic fields in shafts in connection with the measurement of torques.
The problem with static external magnetization may become particuarly serious since static magnetization, by remanence in the transducer material, may give rise to permanent changes in the function of the transducer.
The present invention suggests a method of protecting against the effects of static and/or quasi-static magnetic fields. In addition, the method provides information as to when this protection, in spite of all, is not sufficient and may warn that the transducer does not function satisfactorily, which is very important when the transducer is used in automatic control systems.